Exhaust valve for an internal combustion engine, and a method of strengthening an annular valve seat area in an exhaust valve

ABSTRACT

An exhaust valve for an internal combustion engine comprises a valve disc having a first side ( 14 ) and a second side ( 16 ), a valve stem extending from the first side of the valve disc, and an annular valve seat area ( 11 ) located at the first side of the valve disc. The annular valve seat area ( 11 ) is of a nickel-base alloy or a chromium-base alloy and has a valve seat surface. The annular valve seat area has at least a first annular portion ( 43 ) located in between two second annular portions ( 42 ). The first annular portion has lower hardness than the two second annular portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority based on Danish Patent Application No.PA 2014 70184 filed on Apr. 8, 2014, the contents of all of which areincorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates to an exhaust valve for an internalcombustion engine, the exhaust valve comprising a valve disc having afirst side and a second side, a valve stem extending from the first sideof the valve disc, and an annular valve seat area located at the firstside of the valve disc, which annular valve seat area is of anickel-base alloy or a chromium-base alloy and has a valve seat surface.

Such an exhaust valve is disclosed in U.S. Pat. No. 6,295,731 where avalve blank has a valve disc of smaller diameter than the disc of thecompleted valve and excess material above the valve seat. The excessmaterial is shaped as an isosceles-triangle-sectioned lobe on the valvedisc covering the complete area of the valve seat. The valve blank isheated to a temperature of 530 to 600° C. and is forged in a die havingthe shape of the completed valve disc. During forging theisosceles-triangle-sectioned lobe is plastically deformed and the outerdiameter of the valve disc is increased. The forging of the excessmaterial thus causes plastic flow of all the material at the outer areaof the valve disc.

EP 0 521 821 B1 discloses an exhaust valve comprising a valve dischaving a first side provided with an extending valve stem and an annularvalve seat area with a valve seat surface. The annular valve seat areais of a nickel-base alloy or a chromium-base alloy. Forging may be usedto globally shape the valve seat area. Hot deformation is recommended inorder to distribute carbide formations within the material in order toobtain improved corrosion resistance.

The book ‘Diesel engine combustion chamber materials for heavy fueloperation’ published in 1990 by The Institute of Marine Engineers,London, collected the experience gained from the whole industry andprovided the general conclusion that the valve seat area of an exhaustvalve must have high hardness.

WO 97/47862 discloses an exhaust valve disc having a base body ofaustenitic stainless steel and a valve seat area of a nickel-basematerial provided by welding or by a HIP process. The yield strength ofthe valve seat area may be increased by cold-working of the material,such as by rolling or forging the valve seat area.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to improve the manufacturing ofthe valve seat area.

With a view to this, the initially mentioned exhaust valve according tothe present invention is characterized in that the annular valve seatarea has at least a first annular portion located in between two secondannular portions, which first annular portion has lower hardness thanthe two second annular portions.

The first annular portion of lower hardness has higher ductility thanthe two second annular portions, and the location of the first annularportion in between the two second annular portions in the valve seatarea allows elastic valve seat deformation to occur more in the firstannular portion than in the second annular portions. The harder and lessductile second annular portions may thus be less affected by loadscaused by temperature changes when the engine initiates operation or istaken out of operation or is significantly changing engine load. Inparticular for main propulsion engines in a vessel the engine istypically operated at almost constant engine load—typically full engineload—for several or many days in a row while sailing across seas fromone destination to another, and then operated at low engine load whilethe vessel approaches and enters a port. Such changes in load andresulting changes in operating temperatures affect the valve disc andthe valve seat area. The valve seat area is part of the first side ofthe valve disc and is thus facing away from the combustion chamber inthe engine cylinder. The valve seat area has a conical outer surface anddue to the resulting changes in thickness of the valve disc at the valveseat area the loadings caused by thermal changes of the valve disc areof significance. The first annular portion in the valve seat areareduces the amount of high hardness alloy material in the valve seatarea, as the high hardness alloy material is localised in the two secondannular portions.

In the prior art exhaust valves the entire valve seat area is subjectedto plastically deformation in order to improve the hardness or yieldstrength of the valve seat alloy. The first annular portion of lowerhardness requires that plastic deformation is avoided, or not performedto the same extent as in the second annular portions. As less plasticdeformation is required, the manufacturing of the valve seat area isfacilitated and the plastic deformation carried out may be more precisebecause the volumes of material to be deformed are smaller.

In an embodiment the average hardness at the valve seat surface at themiddle of the first annular portion is at least 80 HV lower than theaverage hardness at the middle of the surface of at least one of thesecond annular portions. Although advantages are obtained also at lowerdifferences of average hardness, such as a difference of at least 50 HV,the difference of at least 80 HV provides a suitable ductility in thefirst annular portion.

In an embodiment the material of the two second annular portions has acrystallographic grain structure of cold deformation and age hardeningbelow recrystallization temperature. The age hardening is not mandatory,but presents an advantageous manner of improving the hardness of the twosecond annular portions when the alloy of the valve seat area is capableof age hardening.

In a further embodiment the material of the first annular portion has acrystallographic grain structure without cold deformation. In thisembodiment the ductility of the material of the first annular portion isabout as high as possible for the given alloy and geometry of the valveseat area.

It is possible to make the entire valve disc of the same alloy as thealloy in the valve seat area. For exhaust valves having largedimensions, such as an outer diameter of the valve disc of 0.10 m ormore, it may be an advantage to use a base body of one alloy and providethis base body with a valve seat area of another alloy. In an embodimentthe valve disc carrying the annular valve seat area comprises a basebody of stainless steel. The stainless steel provides suitably highstrength and the stainless steel alloys are readily available.

In an embodiment the valve disc comprises at its second side a layer ofa hot corrosion resistant material. The second side of the valve discfaces the combustion chamber and is thus subjected to high temperaturesin operation and also to corrosive combustion residues adhering to thesurface of the disc. The layer of hot corrosion resistant material mayimprove the life time of the exhaust valve.

In another aspect, the present invention relates to a method ofstrengthening an annular valve seat area in an exhaust valve for aninternal combustion engine, which exhaust valve comprises a valve dischaving a first side and a second side, a valve stem extending from thefirst side of the valve disc, and an annular valve seat area located atthe first side of the valve disc.

In order to improve the manufacturing of the valve seat area, at leastthe following steps are performed according to the present invention:

a) a valve disc with a raw seat area is provided, which raw seat areahas at least two annular protrusions separated by an intermediateannular area,

b) the two annular protrusions are plastically deformed,

c) the raw seat area is machined to the dimensions of the annular valveseat area,

d) the valve disc with the annular valve seat area is subjected to heattreatment involving at least an aging heat treatment.

The separation of the two annular protrusions by the intermediateannular area reduces the area available for the annular protrusions, andthey consequently have much less volume than the above-mentioned priorart lobe, and the deformation in step b) thus requires less forces. Thedeformation of the protrusions in step b) and the machining in step c)form the two second annular portions in the valve seat area, and thefirst annular portion is located in or formed by the intermediateannular area.

The smaller size of the two annular protrusions also reduces the needfor excess material in the raw seat area, and the valve disc with theraw seat area can be shaped closer to the final dimensions of the valvedisc and thus alloying material is saved and the need for machining instep c) is also reduced.

The plastic deformation can be effected by forging, but preferably theplastic deformation in step b) involves rolling with a cylindricallyshaped roll. The cylindrically shaped roll is in contact with only asmall portion of the annular protrusion at a time because the roll isarranged during rolling with its rotational axis extending in adirection approximately orthogonal to the circular path of the annularprotrusion. It is a distinct advantage that the complete width of theannular protrusion is rolled simultaneously by the roll and that theroll extends to both sides of the annular protrusion because theplastically deformed material cannot just be pressed aside into a ridgebut is pressed into the material of the valve seat area below the roll.It is possible to roll the raw seat area into the shape of the completedvalve seat area and then perform the machining in step c) as a grinding.

As the annular protrusions are separated by the intermediate annulararea the one annular protrusion is located at a larger distance from thestem than the other annular protrusion. It is naturally possible to useonly a single roll for the annular protrusions and to roll bothprotrusions in a common rolling operation, or to roll the one protrusionfirst, and then roll the other protrusion. It is also possible to use aseparate cylindrically shaped roll for each annular protrusion whichallows the roll located at the larger distance to rotate at a slightlyhigher speed than the other roll.

When two rolls are used the separate cylindrically shaped rolls arepreferably carried by a common rolling tool, but are allowed to rotateduring rolling with mutually different rotational speeds.

The individual annular protrusion provided in step a) may have differentshapes, such as a shape with a curved central portion. The top of thecurve can be located at a radial position where the second annularportion should have the highest hardness.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

Examples of embodiments will be described in further detail in thefollowing with reference to the schematic drawings, on which

FIG. 1 illustrates a vertical section through a cylinder of an internalcombustion engine,

FIG. 2 depicts an exhaust valve according to the present invention,

FIG. 3 is a cross sectional view of a valve seat raw area,

FIG. 4 illustrates a prior art rolling method for a valve seat area,

FIG. 5 is a cross sectional view through the valve seat raw area afterapplying the prior art rolling method of FIG. 4,

FIG. 6 illustrates the hardness distribution in the completed valve seatarea manufactured with the prior art rolling method of FIG. 4,

FIG. 7 illustrates a section through an outer area of the exhaust valvedisc and a stationary valve housing part,

FIG. 8 illustrates a section through a first embodiment of raw seat areahaving two annular protrusions separated by an intermediate annular areain accordance with the present invention,

FIG. 9 is an illustration of a rolling tool and a valve disc for themethod according to the present invention,

FIG. 10 illustrates a vertical cross section through a valve seat areain according to the present invention,

FIG. 11 illustrates the hardness distribution in a rolled valve seatarea treated with the method of the present invention, and

FIG. 12 illustrates a section corresponding to FIG. 8 through a secondembodiment of raw seat area in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

When alloys are specified or alloying components mentioned in thepresent description, components are stated in percent by volume (vol-%)but only denoted %.

An exhaust valve 1 is viewed in FIG. 1 in an open position allowingintake air to be supplied from a turbocharger 2 to a scavenge airreceiver 3 via a scavenge air cooler 4 and a water mist catcher 5. Theintake and scavenge air flows from the scavenge air receiver through arow of scavenge air ports 6 in the lower end of a cylinder 7 and upwardsthrough the cylinder in a swirling motion and up towards the upperportion of the cylinder, while at the same time pressing out hotcombustion gasses via the open exhaust valve and into the exhaust gasreceiver 8. The flow of hot combustion gasses past the valve seats ofthe exhaust valve can occur at high gas speeds, in particular when thevalve opens and closes.

The exhaust valve 1 is mounted in an exhaust valve housing 9 that isfixed to a cylinder cover 10 at the top of the cylinder. In the closedposition of the exhaust valve, a valve seat area 11 on the upper side ofa valve disc 12 abuts a corresponding valve seat on a stationary part13, also called the bottom piece, of the valve housing.

The valve disc has a first side 14 facing upwards towards exhaustchannel 15 and a second side 16 facing downwards to combustion chamber17 in the cylinder 7. A valve stem 18 extends centrally from the firstside 14 and has a portion 19 for mounting of valve rotator wings, abearing area 20 for positioning in a stationary valve guide in the valvehousing, a groove 21 for mounting of an air spring piston or a springend support, and an upper end area 22 for mounting at an actuator pistonin a hydraulic valve actuator. The upper end of the stem mayalternatively be actuated by a cam in a traditional manner.

The stationary part 13 can be cooled by water. In the mounted positionof the exhaust valve, the stem 18 extends upwards from the valve discand passes the exhaust channel and continues upwards through the valveguide and up into an exhaust valve actuator. A piston of a pneumaticspring is mounted to the outside of the stem. A hydraulic actuatorpiston at the top of the stem can activate the exhaust valve fordownward movement, and the pneumatic spring acts in the oppositedirection and closes the exhaust valve when the pressure in thehydraulic actuator is relieved. The embodiment illustrated in FIG. 2 isan exhaust valve for an engine of one of the types ME or MC ofapplicant's brand. The exhaust valve could also be mechanically actuatedin well-known manner, and a mechanical return spring could also be used.The portion of the stem located in the exhaust channel is provided withoblique fins extending out radially from the stem. These fins cause theexhaust valve to rotate some degrees in the circumferential directionwhen the exhaust gas is flowing out. In another embodiment the stem doesnot have such fins.

The exhaust valve can be for a four-stroke internal combustion engine,or it can be for a two-stroke internal combustion engine, preferably alarge two-stroke crosshead engine, which may have cylinder diametersranging from 250 to 1100 mm. The outer diameter of the valve disc 12 isin the range from 100 mm to 600 mm, depending on the cylinder bore, whenthe valve is for such large two-stroke engines. The internal combustionengine utilizing the exhaust valve spindles may be of the make MANDiesel & Turbo, such as of the type MC or ME, or may be of the makeWartsila or Sulzer Diesel, such as of the type RTA of RTA-flex, or maybe of the make Mitsubishi. When the exhaust valve is for a four-strokeengine the outer diameter of the valve disc 12 is typically in the rangefrom 50 mm to 300 mm.

Valve seat area 11 is in general annular and conical and located next toan outer end portion of the valve disc. The valve seat area can be ofthe same alloy as the remainder of the exhaust valve, or it can be of adifferent alloy deposited in a groove 23 on a base body 24, asillustrated in FIG. 3. In FIG. 3 groove 23 is filled with six weld seams25. It is known to plastically deform such a deposited valve seat areaby use of a rolling tool 26 having a roll with a V-shape outer surface.The V-shape is pressed into the material of the valve seat in severalrounds at different diameters from the central axis of the exhaustvalve. After rolling the cross sectional shape of the valve seat area isas depicted in FIG. 5. The rolling presses material sideways of the rollso that ridges 27 are formed. The rolling depth 28 below the outersurface of the raw seat represents excess material that must be removedafter rolling, such as by machining on a lathe.

A reference sample was manufactured of an Inconel 718 valve seat alloywelded onto a stainless steel base body. Rolling was performed asillustrated in FIG. 4 with the results as presented in FIG. 5. Theexcess material was removed by turning on a lathe. The valve disc wassubjected to age hardening. The hardness of the resulting valve seatarea is illustrated in FIG. 6. The following hardness values have beenachieved: areas marked 29 hardness 575 HV; areas marked 30 hardness 550HV; areas marked 31 hardness 525 HV; areas marked 32 hardness 500 HV;areas marked 33 hardness 475 HV; and areas marked 34 hardness 450 HV.

An example of a stationary valve housing part 13 is illustrated in FIG.7. A section is shown through an outer area of the exhaust valve discand a stationary valve housing part having seat surfaces 35, 36 abuttingthe valve seat area 11 when the exhaust valve is in closed position.Alternatively the stationary valve housing part can have a single seatsurface substantially covering the outer surface of the valve seat area.

An example of the method of strengthening an annular valve seat areaaccording to the present invention is described in the following. Thevalve disc 12 with a raw seat area is provided. The raw seat area hastwo annular protrusions 37 separated by an intermediate annular area 38.The annular protrusions 37 have less width than the intermediate annulararea 38. The two annular protrusions 37 are then plastically deformed byrolling with a rolling tool 39 as illustrated in FIG. 9. The rollingtool has a roll 40 which is cylindrically shaped and mounted in the toolso that the roll can rotate about central axis 41, which extends inparallel with the cylindrical outer surface of the roll. During rollingthe rolling tool presses the roll against the seat raw area withsufficient force to cause plastic deformation of the annularprotrusions. A load of 5 to 10 t on the roll may be suitable forgenerating the required force for rolling a seat area on a valve dischaving an outer diameter of about 0.35 m. During rolling the rollingtool is positioned with central axis 41 in parallel with the outersurface of the valve seat area 11, and the valve disc is fixed on arotating table. The rotating table rotates the valve disc about thelongitudinal axis of the valve and thus the roll 40 rolls on the valveseat area. The rolling tool may also be embodied with two rolls bothrotating about axis 41, and the rolling tool may then be positioned sothat the one roll is pressing only on one of the annular protrusions,and the other roll is pressing only on the other of the annularprotrusions.

After completion of the rolling the annular protrusions have beenplastically deformed into the valve seat area so that the outer surfaceof the valve seat area 11 follows a conical surface. The individualannular protrusion 37 has thus been deformed into a second annularportion 42 of the valve seat area, and a first annular portion 43 islocated in between the second annular portions 42. The first annularportion 43 has lower hardness than the two second annular portions 42because the first annular portion has not been subjected to the sameamount of plastic deformation as the second annular portions.

The hardness distribution in the valve seat area is depicted in FIG. 11for an example of a valve disc having an outer diameter of 0.35 m and abase body of stainless steel (SNCrW stainless steel) and a valve seatarea of alloy Inconel 718 (Inconel is a trade mark of Special MetalsCorporation) of 53% nickel+cobalt, 19% chromium, 18% iron, 5.3% niobium,3% molybdenum, 0.9% titanium, 0.5% aluminium, less than 1% cobalt, lessthan 0.08% carbon, less than 0.35% manganese, less than 0.35% silicon,less than 0.015% phosphorus, less than 0.015% sulfur, less than 0.006%boron, less than 0.3% copper, and unavoidable impurities. Followingdeposition of the alloy by welding in the valve seat area, the outersurface was turned in a lathe to the profile illustrated in FIG. 8 wherethe annular protrusion 37 had a height h of 1.0 mm and an angling of thesides of α=50° and a curved central portion having a radius of curvatureof 0.5 mm. The rolling was performed with a loading of 10 t on therolling tool. After rolling the valve disc was subjected to an agingheat treatment at 720° C. for six hours followed by 620° C. for sixhours. As an alternative the aging heat treatment may be performed for10 hours at 730° C. A piece was cut from the valve seat area and thehardness was measured. The hardness distribution is illustrated in FIG.11, and the values are given in HV in the following table 1.

TABLE 1 Reference Vickers hardness numeral range HV 44 450 to 588 45 429to 450 46 408 to 450 47 387 to 408 48 366 to 387 49 345 to 366 50 325 to345 51 304 to 325 52 283 to 304 53 262 to 283 54 241 to 262 55 220 to241 56 200 to 220 57 below 200

The area of highest hardness of 588 HV was located at the middle of thesecond annular portion at about 0.3 mm below the outer surface. In orderto complete the valve seat area, the outermost material is removed to adepth of 0.2 mm as indicated by the broken line in FIG. 11.

The annular protrusions 37 in the raw seat area on valve disc 12 mayhave other configurations, such as the one illustrated in FIG. 12 wherethe central portion of the protrusion 37 is flat. Hardness distributionsin the valve seat area have been found for some different embodiments ofthe protrusions. The height h of the protrusion and the angle α for thesides of the protrusion are illustrated in FIG. 12.

Examples having an angling of the sides of α=30° and a curved centralportion having a radius of curvature of 1.0 mm were made having theheights h of 0.8 mm, 1.0 mm and 1.5 mm. The protrusion having a heightof 0.8 mm provided a hardness of 504 HV and a slightly lower hardness ofabout 450 HV at the middle of the surface of the second annular portion.The protrusion having a height of 1.0 mm provided a hardness of 502 HVand a slightly lower hardness of about 475 HV at the middle of thesurface of the second annular portion. The protrusion having a height of1.5 mm provided a hardness of 503 HV and a quite even hardness acrossthe width of the surface of the second annular portion. Examples havingan angling of the sides of α=50° and a curved central portion having aradius of curvature of 1.0 mm were made having the heights h of 1.0 mm,1.5 mm and 2.0 mm. The protrusion having a height of 1.0 mm provided ahardness of 575 HV and a slightly lower hardness of about 450 HV at themiddle of the surface of the second annular portion. The protrusionhaving a height of 1.5 mm provided a hardness of 599 HV and a quite evenhardness of about 500 HV across the width of the surface of the secondannular portion. The protrusion having a height of 2.0 mm provided ahardness of 619 HV and a quite even hardness of about 520 HV across thewidth of the surface of the second annular portion. Two examples havinga flat central portion of a width of 0.5 mm and a height of 1.0 mm weremade with anglings of the sides of α=50° and 60°. The example havingα=50° provided a hardness of 588 HV and a quite even hardness of about525 HV across the width of the surface of the second annular portion.The example having α=60° provided a hardness of 619 HV and a quite evenhardness of about 530 HV across the width of the surface of the secondannular portion.

In all examples the hardness of the first annular portion was much lowerthan the hardness of the second annular portions. The hardness of thefirst annular portion was generally in the range from 200 HV to 300 HV,and in most examples the hardness of the first annular portion wasgenerally in the range from 200 HV to 260 HV. As the ductility of thealloy material increases when the hardness decreases, the first annularportion is thus providing the valve seat area with a significantlyincreased ductility. If the stationary valve housing part has two seatsurfaces 35, 36 the second annular portions 37 are preferably located soas to abut seat surfaces 35, 36 when the valve is in closed position. Ifthe stationary valve housing part has a single, wide seat surface thesecond annular portions 37 are preferably located so as to both abutthis seat surface when the valve is in closed position. It is preferredthat the average hardness at the valve seat surface at the middle of thefirst annular portion is at least 80 HV lower than the average hardnessat the middle of the surface of at least one of the second annularportions. This average hardness is found by measuring the hardness atleast at five positions distributed along the circle located at themiddle of the surface of the portions, and then finding the average ofthe measured values. The difference in average hardness may be only 50HV or may be larger than 80 HV, such as 150 HV, 250 HV or 300 HV. Inmost of the above-mentioned examples the hardness of the first annularportion was generally in the range from 200 HV to 260 HV, and thehardness of the second annular portions was generally higher than 460HV, which makes the difference at least 200 HV.

Other alloys than Inconel 718 may be used as the alloy of the valve seatarea. The valve seat area can be of a nickel-based alloy, which alloystated in percent-by-weight and apart from common impurities andinevitable residual amounts of deoxidizing components comprises at leastfrom 34.0 to 44.0% Cr, an aggregate amount of Nb and Ta in the rangefrom at least 2.8 to 6.1%, from 0.3 to 2.0% Ti, a balance of Ni, andoptionally one or more of the components at the most 0.2% Al, at themost 0.04% B, at the most 0.8% Fe, at the most 0.04% C, and at the most0.4% Si. As a further example the valve seat area can be of anickel-based alloy comprising at least the components 10-25% Cr, 5-25%Co, 2-23% Ta, a balance of Ni, and optionally one or more of thecomponents up to 10% Mo+W, up to 11% Nb, up to 4% Ti, up to 3% Al, up to0.3% C, up to 1% Si, up to 0.015% S, up to 5% Fe, and up to 3% Mn. As afurther example the valve seat area can be of a nickel-based alloycomprising at least the components 20-23% Cr, 8-10% Mo, 3.15-4.15%Ta+Nb, a balance of at least 58% Ni, and optionally one or more of thecomponents up to 5% Fe, up to 0.1% C, up to 0.5% Mn, up to 0.5% Si, upto 0.4% Al, up to 0.4% Ti, up to 1.0% Co, up to 0.015% Si, and up to0.015% P. As a further example the valve seat area can be of anickel-based alloy comprising at least the components 27-30% Cr, 7-11%Fe, up to 0.05% C, optionally small amounts of Mg, Co, Si and a balanceof at least 58% Ni. As a further example the valve seat area can be ofan alloy comprising at least the components 10-25% Cr, 3-12% Nb and/orTa, 5-25% Fe, and a balance of nickel and/or cobalt. As a furtherexample the valve seat area can be of an alloy comprising at least thecomponents 0.04-0.08% C, 46-49% Cr, 0.3-0.5% Ti, and a balance of Ni. Asa further example the entire valve disc, and thus also the valve seatarea can be of the alloy Nimonic 80A (Nimonic is a trade mark of SpecialMetals Corporation) comprising at least the components 18-21% Cr,1.8-2.7% Ti, 1.0-1.8% Al, at the most 3.0% Fe, at the most 2.0% Co, andas balance Ni.

When the valve disc comprises a base material of stainless steel it canbe a stainless steel, such as an alloy having 0.25% C, 1.4% Si, 1.3% Mn,20% Cr, 9% Ni, 3% W, and the balance Fe. Other stainless steels can alsobe used and are well-known in the art of exhaust valves.

In an embodiment the valve disc 12 comprises at its second side a layerof a hot corrosion resistant material, such as a layer of a nickel-basedalloy comprising at least the components 20-23% Cr, 8-10% Mo, 3.15-4.15%Ta+Nb, a balance of at least 58% Ni, and optionally one or more of thecomponents up to 5% Fe, up to 0.1% C, up to 0.5% Mn, up to 0.5% Si, upto 0.4% Al, up to 0.4% Ti, up to 1.0% Co, up to 0.015% Si, and up to0.015% P.

In embodiments where the valve seat area is of an alloy different fromthe alloy of the base body of the exhaust valve, the valve seat area canbe deposited in a groove 23 on a base body 24 by welding with severalweld seams. The welding can be performed by plasma welding, laserwelding, gas metal arc welding or gas tungsten arc welding. The valveseat area can alternatively be deposited by processes like cold-sprayingor HIP. In embodiments having a layer of a hot corrosion material on thesecond side of the valve disc, the mentioned processes can likewise beapplied to deposit the layer.

Details of the above-mentioned embodiments and examples may be combinedinto further embodiments and examples within the scope of the patentclaims.

1. An exhaust valve for an internal combustion engine, the exhaust valvecomprising a valve disc having a first side and a second side, a valvestem extending from the first side of the valve disc, and an annularvalve seat area located at the first side of the valve disc, wherein theannular valve seat area is of a nickel-base alloy or a chromium-basealloy and has a valve seat surface, wherein the annular valve seat areahas at least a first annular portion located in between two secondannular portions, and wherein the first annular portion has lowerhardness than the two second annular portions.
 2. An exhaust valve asclaimed in claim 1, wherein the average hardness at the valve seatsurface at the middle of the first annular portion is at least 80 HVlower than the average hardness at the middle of the surface of at leastone of the second annular portions.
 3. An exhaust valve as claimed inclaim 2, wherein the material of the two second annular portions has acrystallographic grain structure of cold deformation and age hardeningbelow recrystallization temperature.
 4. An exhaust valve as claimed inclaim 3, wherein the material of the first annular portion has acrystallographic grain structure without cold deformation.
 5. An exhaustvalve as claimed in claim 4, wherein the valve disc at its second sidecomprises a layer of a hot corrosion resistant material.
 6. An exhaustvalve as claimed in claim 5, wherein the valve disc carrying the annularvalve seat area comprises a base body of stainless steel.
 7. An exhaustvalve as claimed in claim 1, wherein the material of the two secondannular portions has a crystallographic grain structure of colddeformation and age hardening below recrystallization temperature.
 8. Anexhaust valve as claimed in claim 7, wherein the material of the firstannular portion has a crystallographic grain structure without colddeformation.
 9. An exhaust valve as claimed in claim 8, wherein thevalve disc at its second side comprises a layer of a hot corrosionresistant material.
 10. An exhaust valve as claimed in claim 9, whereinthe valve disc carrying the annular valve seat area comprises a basebody of stainless steel.
 11. An exhaust valve as claimed in claim 1,wherein the material of the first annular portion has a crystallographicgrain structure without cold deformation.
 12. An exhaust valve asclaimed in claim 11, wherein the material of the two second annularportions has a crystallographic grain structure of cold deformation andage hardening below recrystallization temperature.
 13. An exhaust valveas claimed in claim 12, wherein the valve disc at its second sidecomprises a layer of a hot corrosion resistant material.
 14. An exhaustvalve as claimed in claim 13, wherein the valve disc carrying theannular valve seat area comprises a base body of stainless steel.
 15. Anexhaust valve as claimed in claim 1, wherein the valve disc at itssecond side comprises a layer of a hot corrosion resistant material. 16.An exhaust valve as claimed in claim 15, wherein the valve disc carryingthe annular valve seat area comprises a base body of stainless steel.17. An exhaust valve as claimed in claim 15, wherein the material of thefirst annular portion has a crystallographic grain structure withoutcold deformation.
 18. A method of strengthening an annular valve seatarea in an exhaust valve for an internal combustion engine, said exhaustvalve comprising a valve disc having a first side and a second side, avalve stem extending from the first side of the valve disc, and anannular valve seat area located at the first side of the valve disc,wherein at least the following steps are performed: a) a valve disc witha raw seat area is provided, said raw seat area having at least twoannular protrusions separated by an intermediate annular area, b) thetwo annular protrusions are plastically deformed, c) the raw seat areais machined to the dimensions of the annular valve seat area, d) thevalve disc with the annular valve seat area is subjected to heattreatment involving at least an aging heat treatment.
 19. A method ofstrengthening an annular valve seat area in an exhaust valve as claimedin claim 18, wherein the plastic deformation in step b) involves rollingwith a cylindrically shaped roll.
 20. A method of strengthening anannular valve seat area in an exhaust valve as claimed in claim 19,wherein a separate cylindrically shaped roll is provided for eachannular protrusion.
 21. A method of strengthening an annular valve seatarea in an exhaust valve as claimed in claim 18, wherein the separatecylindrically shaped rolls are carried by a common rolling tool, but areallowed to rotate during rolling with mutually different rotationalspeeds.
 22. A method of strengthening an annular valve seat area in anexhaust valve as claimed in claim 18, wherein the individual annularprotrusion provided in step a) has a curved central portion.